In general, a lithographic printing plate is composed of an oleophilic image area accepting ink and a hydrophilic non-image area accepting dampening water in the process of printing. Lithographic printing is a printing method which comprises rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink unreceptive area), thereby making a difference in adherence of ink on the surface of the lithographic printing plate, and depositing the ink only on the image area by utilizing the nature of water and printing ink to repel with each other, and then transferring the ink to a printing material, for example, paper.
In order to prepare the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (also referred to as a photosensitive layer or an image-recording layer) has heretofore been broadly used. Ordinarily, the lithographic printing plate is obtained by conducting plate making according to a method of exposing the lithographic printing plate precursor through an original, for example, a lith film, and then removing the unnecessary portion of the image-recording layer by dissolving with an alkaline developer or an organic solvent thereby revealing the hydrophilic surface of support to form the non-image area while leaving the image-recording layer for forming the image area.
Thus, in the hitherto known plate making process of lithographic printing plate precursor, after exposure, the step of removing the unnecessary portion of the image-recording layer by dissolving, for example, with a developer is required. However, in view of the environment and safety, a processing with a developer closer to a neutral range and a small amount of waste liquid are problems to be solved. Particularly, since disposal of waste liquid discharged accompanying the wet treatment has become a great concern throughout the field of industry in view of the consideration for global environment in recent years, the demand for the resolution of the above-described problems has been increased more and more.
On the other hand, digitalized technique of electronically processing, accumulating and outputting image information using a computer has been popularized in recent years, and various new image outputting systems responding to the digitalized technique have been put into practical use. Correspondingly, attention has been drawn to a computer-to-plate (CTP) technique of carrying digitalized image information on highly converging radiation, for example, laser light and conducting scanning exposure of a lithographic printing plate precursor with the light thereby directly preparing a lithographic printing plate without using a lith film. Thus, it is one of important technical subjects to obtain a lithographic printing plate precursor adaptable to the technique described above.
As described above, the decrease in alkali concentration of developer and the simplification of processing step have been further strongly required from both aspects of the consideration for global environment and the adaptation for space saving and low running cost. However, since hitherto known development processing comprises three steps of developing with an aqueous alkali solution having pH of 11 or more, washing of the alkali agent with a water-washing bath and then treating with a gum solution mainly comprising a hydrophilic resin as described above, an automatic developing machine per se requires a large space and problems of the environment and running cost, for example, disposal of the development waste liquid, water-washing waste liquid and gum waste liquid still remain.
In response to the above situation, for instance, a developing method of processing with a developer having pH of 8.5 to 11.5 and a dielectric constant of 3 to 30 mS/cm and containing an alkali metal carbonate and an alkali metal hydrogen carbonate is proposed in JP-A-11-65126 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”). However, since the developing method is required a water-washing step and a treatment step with a gum solution, it does not resolve the problems of the environment and running cost.
Also, processing with a processing solution having pH of 11.9 to 12.1 and containing a water-soluble polymer compound is described in the example of EP-A-1868036. However, since the printing plate obtained by the processing is left in the state that the alkali of pH 12 adheres on the surface thereof, a problem in view of safety of an operator arises and with the lapse of long time after the preparation of the printing plate until the initiation of printing, the image area gradually dissolves to result in deterioration in printing durability or ink-receptive property. In JP-T-2007-538279 (the term “JP-T” as used herein means a published Japanese translation of a PCT patent application), processing with a processing solution having pH of 3 to 9 and containing a water-soluble polymer compound is described. However, since the processing solution does not contain a basic component, it is necessary to enable development by making a polymer used in a photosensitive layer hydrophilic and thus, a problem occurs in that printing durability severely degrades.
On the other hand, in order to achieve good compatibility between developing property and printing durability, a technique of using a binder polymer in which an acid group is connected to a polymer chain with a straight chain connecting group to increase dispersion property in a developer and alkali-solubility (alkali-responsiveness) in an aqueous solution is proposed as described in JP-A-2004-318053 and JP-A-2005-250158. However, in a low alkali range, specifically, in a region of pH of 11 or lower, a problem in that the developing property severely degraded may arise even when such a binder polymer is used, because of decrease in the alkali-responsiveness.